PROJECT SUMMARY/ABSTRACT Contrast agents are relied upon in MR molecular imaging to provide positive or negative contrast to reveal biologically specific structures and processes in vivo. The detection sensitivity of the pre-post injection contrast changes is difficult against a complex image background. This is a major limiting factor to the clinical deployment of targeted contrast agent, since insensitive detection translates to the need for large doses, or highly concentrated molecular targets. The problem can be attributed to biological 1/f fluctuations in image intensity between the pre and post injection scan that occur in the 5-30 min between the images. Other essential considerations in a contrast agent acquisition includes the appearance of the agent as a ?positive? or ?negative? contrast against the background and the ability of the acquisition to distinguish the bound and un- bound pools of a targeted contrast agent. We will address each of these limitations by developing two classes of novel methods to externally modulate how the contrast agent affects the image signal prior to each k-space acquisition. The first class aims to use a recently developed spin-locking sequence that is resonantly sensitive to either acoustic waves or the non-linear response of iron-oxide nanoparticles. The effect is modulated ?on? or ?off? by simply altering the external acoustic wave or applied drive field on or off resonance. The spin-locking sequence has the ability to control the appearance of the agent as either ?positive? contrast or ?negative? contrast. It defeats the effect of 1/f noise by rapid modulation and provides a statistical framework to characterize the detection uncertainty. The second class of modulation is designed to add discrimination between bound and unbound pools of the targeted contrast agent. We propose a field cycling approach, whereby the applied field modulates the agents' relaxivity during a preparation phase in a different way for bound and unbound pools. We will demonstrate the ability of MR gradient coils to perform this role using the MGH 300 mT/m ?Connectome? gradient. This method thus has tremendous potential to quantify the bound pool of targeted contrast agents in humans, and will enable important new applications in biomedical imaging that can be immediately utilized with certain FDA-approved contrast agents. The Martinos Center at the Massachusetts General Hospital is one of the largest imaging centers in the world, and features the ideal environment and infrastructure needed to complete the proposed research strategy. The Center has extensive hardware and computing resources, several large bore and small bore (animal) MR scanners, as well as dedicated molecular imaging, chemistry, and animal surgery laboratories that will facilitate my development of the proposed novel contrast agent MR acquisitions. Furthermore, Drs. Lawrence Wald and Peter Caravan have the expertise in MR molecular imaging and pulse sequence development, as well as experience in mentorship, that will help guide this project and my training.